In recent years, a method which has been widely used as a production method for nitrogen generates high purity nitrogen by a PSA method by making use of an adsorbent which preferentially adsorbs oxygen from a mixed gas of oxygen and nitrogen, e.g., air (“air” will be used below to illustrate a mixed gas of oxygen and nitrogen).
This is a technique of separating nitrogen from air by means of adsorption based on a PSA method. This technique comprises: a pressurization adsorption step of pressurizing air to a suitable pressure and bringing the air into contact with an adsorbent layer, selectively adsorbing the pressurized oxygen portion, and maintaining this for a certain period, an adsorption step of terminating the supply of air to this adsorbent layer after the adsorbent has been saturated with oxygen; and a depressurization regeneration step, after completion of the adsorption step, of decreasing the pressure to the atmospheric pressure by depressurizing the adsorbent layer, desorbing the oxygen portion which has been adsorbed in the adsorbent by this means, and regenerating the adsorbent.
MSC is activated carbon which literally has a molecular sieving action, and is characterized in that, in comparison with normal activated carbon, the average pore size is smaller and the pore distribution is sharp. Since the pore size and molecular diameter of the adsorbate are very similar, MSC experiences a phenomenon in which the adsorption rate is decreased for specific combinations of the adsorbent and the adsorbate.
For example, when attempting to obtain nitrogen-enriched gas from a mixed gas of oxygen and nitrogen such as air, it is preferred that the MSC be one for which the adsorption rate of nitrogen with respect to the adsorption rate of oxygen is very slow due to the difference in the molecular diameters of oxygen and nitrogen.
Such a PSA method which carries out adsorption and desorption by changing the pressure has the characteristic that it is possible to make the air processing amount per unit adsorbent large since it is possible to change the pressure in a relatively short cycle of about one minute. Therefore, in comparison with a conventional nitrogen separation method carried out by a cryogenic air separator and adopted as a nitrogen production method, since the apparatus structure is greatly simplified and there are also advantages in terms of the decrease in cost for setting up the apparatus, it has come to be very widely used in small and medium scale businesses which use nitrogen.
Furthermore, the production process in the nitrogen production technique carried out by a PSA method which uses an MSC adsorbent is primarily constituted by a pressure increasing step, adsorption step, pressure-equalization depressurization step, depressurization step, regeneration step, and pressure-equalization pressurization step.
In contrast, regarding the operating characteristics of the apparatus, as disclosed in the magazine “Kagaku Sochi (Chemical Apparatus)” 1983 (Showa 58), No. 8, page 39, etc., a large number of studies have come about from the prior art. Recently, as a result of improvements and advances in processes, progress can be seen in technologies in which production of nitrogen having a high purity of 99.999% is possible; yet, on the other hand, the provision of cheaper nitrogen or a decrease in production costs has been strongly desired.
The separation processing efficiency in the PSA method is largely dependent on the adsorption performance of MSC, which is the separation adsorbent, and on the process of the PSA method, and there is a necessity for improvements in the adsorption characteristics of the MSC adsorbent in the large reduction of production costs and further improvements of the process of the PSA method.
Regarding advances in the process of the nitrogen production method by a PSA method, the publication of Japanese Unexamined Patent Application, First Publication No. Hei 8-224428, for example, discloses that by carrying out a countercurrent emission operation in a direction facing the introduction direction of air during the adsorption step either prior to the pressure-equalization step which occurs after completion of the adsorption step or at the same time as the pressure-equalization step, it is possible to improve the purity of the nitrogen product which is collected in the adsorption step. However, this is little more than an improvement of the yield and productivity which are taken as the performance of the PSA method.
Furthermore, the publication of Japanese Unexamined Patent Application, First Publication No. Hei 10-192636 discloses that by removing gas remaining in a column upon completion of an adsorption step from an intermediate position of an adsorption column in which an adsorbent is filled after the completion of the adsorption step and recovering gas by introducing it into another adsorption column from an air introduction end after completion of a regeneration step, it is possible to improve both the nitrogen productivity and yield. However, this is nothing more than the effect in a restricted area of an ultrapure region in which the concentration of oxygen, which is an impurity in the collected nitrogen product, is several ppm.
In contrast, regarding the adsorption characteristics of MSC, the publication of Japanese Unexamined Patent Application, First Publication No. Sho 59-45914, for example, discloses that it is necessary to have an oxygen adsorption amount of 5 ml/g or more and an oxygen/nitrogen selectivity in an amount of 20 to 23 or more in order to efficiently separate oxygen and nitrogen by a PSA method. However, this is not sufficient in terms of satisfying the currently desired reduction in production costs.
Furthermore, the publication of Japanese Unexamined Patent Application, First Publication No. Hei 3-242649 discloses MSC as a starting material in which the majority of pores have a pore size of about 4.5 to 8 angstroms and discloses that it is possible to obtain an MSC adsorbent having improved adsorption characteristics and higher selectivity by thermal decomposition of hydrocarbons in the carbon material using a two-stage processing method. However, regarding the MSC adsorbent obtained here, there is nothing more than the disclosure of simply confining the majority of pore sizes to about 4.0 angstroms.
Moreover, the invention of Japanese Patent No. 2623487 has a structure in which a plurality of spherical particles having a particle size of 0.8 to 120 μm are randomly and three-dimensionally layered and united. By using MSC (molecular sieving carbon) of which the capacity ratio of the adsorption amount of oxygen and nitrogen after one minute is 3.5 to 20 when single component adsorption is carried out at a pressure of 2.5 kgf/cm2, it is possible to generate a large quantity of high purity nitrogen at a low energy consumption rate. Although there is a disclosure regarding the preferable adsorption characteristics of MSC, this relates to a specific MSC in which the primary particles are spherical and which have the characteristic of being spheres in which the oxygen equilibrium adsorption amount is large in comparison with a normally used natural raw material such as coconut shell char.
Furthermore, the publication of Japanese Unexamined Patent Application, First Publication No. Hei 5-269331 discloses that improving the oxygen adsorption amount per adsorbent unit capacity is more effective than the oxygen/nitrogen selectivity of MSC with regard to the performance of the PSA method. By using MSC having a capacity of 9 cc/cc or more as the oxygen adsorption amount per unit capacity, even if the selectivity is 15% lower than that conventionally, it is possible to obtain the same performance. However, there is a problem in that, in exchange for improving the oxygen adsorption amount, the selectivity decreases, and therefore, regardless of the increase of the oxygen adsorption amount, the failure to achieve an improvement of the performance of the PSA method becomes a reality.